In a construction machine, when a pump absorption torque is equal to or lower than a rated engine torque, an engine output torque is matched to the pump absorption torque in a high-speed control area on an engine-output-torque-characteristics line showing a relationship between an engine speed and the engine output torque. For instance, the target engine speed is set corresponding to the setting of a fuel dial and a high-speed control area is determined corresponding to this target engine speed.
Alternatively, the high-speed control area is set corresponding to the setting of the fuel dial and the target engine speed is set corresponding to this high-speed control area. The pump absorption torque and the engine output torque are controlled for matching in this high-speed control area.
Many operators generally set a target engine speed at or around a rated engine speed so as to improve a workload. A low engine-fuel-consumption area (i.e., an engine-fuel-efficient area) usually exists in a middle-speed area and a high-torque area on the engine-output-torque-characteristics line. Accordingly, a high-speed control area defined between a non-load high-idle speed and the rated engine speed does not correspond to an efficient area in terms of fuel consumption.
In order to drive an engine in the fuel-efficient area, a typically known control device presets a value of a target engine speed and a value of a target engine output torque such that the values correspond to each other, for each of plural selectable operation modes (see, for instance, Patent Literature 1). With the use of such a control device, when an operator selects, for instance, a second operation mode, the engine speed can be set lower than that in a first operation mode, and therefore the fuel consumption can be improved.
However, according to the above-described operation mode switching, the operator needs to operate the operation mode switching each time so as to improve the fuel consumption. Further, in a situation where the engine speed in the second operation mode is set at a value simply reduced relative to the engine speed in the first operation mode, selection of the second operation mode leads to the following problem.
The maximum speed of a working device of a construction machine (hereinafter referred to as a working equipment) is decreased as compared to that in the first operation mode. As a result, a workload in the second operation mode becomes smaller than that in the first operation mode.
In order to solve this problem, the applicant has already filed a patent application directed to an engine control device and an engine control method (see Patent Literature 2). According to the above engine control device, when a pump capacity and an engine output torque are low, the drive control of engine is conducted based on the second target engine speed that is closer to a low-speed area than the preset first target engine speed, thereby reaching the preset target engine speed corresponding to the pump capacity of a variable displacement hydraulic pump driven by the engine or the detected engine output torque.
According to the above engine control device, the fuel consumption of the engine is improvable and the engine speed is excellently smoothly changeable while a pump discharge amount required for the working equipment is maintained. Furthermore, an uncomfortable feeling resulting from a discontinuous change in engine noise can be prevented.